Physical Chemistry 2.2 - Reaction Feasibility - Feasability of a Reaction, Gibbs Free Energy and Total Entropy

Question 1

Spontaneous reactions happen without any ___.

Answer 1

input

Question 2

Non-spontaneous reactions ___ happen, but they require…

Answer 2

can, input

Question 3

Any reaction is feasible if…

Answer 3

ΔG is negative

Question 4

Almost all ___ reactions are spontaneous in standard conditions, but they may be very ___.

Answer 4

exothermic, slow (eg iron rusting)

Question 5

The endothermic reverse reaction ___ occurs spontaneously in ___ conditions, but it can happen in ___-___ conditions.

Answer 5

never, standard, non-standard

Question 6

Endothermic ___ can be spontaneous in standard conditions, but ___ are not ___!

Answer 6

changes (eg ice melting), changes are not reactions!

Question 7

Name three factors which affect the spontaneity of a reaction.

Answer 7

enthalpy, entropy, temperature

Question 8

Gibbs free energy (ΔG°) is the energy that can be…

Answer 8

used to do work

Question 9

What can the change in Gibbs free energy (ΔG) be used to predict?

Answer 9

the spontaneity (or feasibility) of a reaction

Question 10

If ΔG is positive, a reaction…
If ΔG is negative, a reaction…

Answer 10

isn’t feasible and will never happen.
is feasible and will happen (however slow it may be)

Question 11

When does a reaction become feasible?

Answer 11

When ΔG = 0.

Question 12

In which two ways can you calculate ΔG?

Answer 12

1. using the standard free energies of the reactants and products:
   ΔG. = ∑G° (products) - ∑ΔG° (reactants)
2. using the change in enthalpy, the temperature and the change in entropy:
   ΔG = ΔH - TΔS
   * both in data booklet

Question 13

For each of the following, state its name and the units it’s measured in.
1. ΔS
2. ΔH
3. ΔG
4. T

Answer 13

1. ΔS - Entropy (measured in J/K/mol - must be converted to kJ/K/mol for calculations)
2. ΔH - Enthalpy (measured in kJ/mol)
3. ΔG - Gibbs Free Energy (measured in kJ/mol)
4. T - Temperature (measured in Kelvin - K)

Question 14

Which equation should be used to find the temperature at which a reaction becomes feasible?

Answer 14

T = ΔH / ΔS
(sub in G=0 to ΔG = ΔH - TΔS and rearrange for T)

Question 15

Because of the whole -TΔS situation, temperature can ___ or ___ to make a reaction feasible.
A feasible reaction is any reaction where ‘ΔH - TΔS’ (aka Δ_) is _ve.

Answer 15

increase, decrease.
ΔG, negative

Question 16

What does G equal at equilibrium?

Answer 16

0.

Question 17

A reversible reaction will proceed spontaneously until…

Answer 17

ΔG = 0.

Question 18

Equilibrium can be reached starting from just the ___ or ___, despite the fact that one of the reactions has a ____ ΔG.

Answer 18

reactants, products, positive

Question 19

How does *this not violate the second law of thermodynamics?
*(Equilibrium can be reached starting from just the reactants or products, despite the fact that one of the reactions has a positive ΔG.)

Answer 19

because the positive ΔG value that was calculated was for the complete conversion of one mole of reactants to products, and (more importantly) the reaction mixture has a higher entropy, and therefore lower ΔG.

Question 20

Since the reaction mixture of an equilibrium has a higher entropy (and therefore __er ΔG) than the reactants or products, reactions to reach equilibrium will always have a ___ ΔG.
This means they’ll be ___!

Answer 20

lower, negative.
Feasible

Question 21

If ΔG is positive, the position of equilibrium will be more towards the ___.
If ΔG is negative, the position of equilibrium will be more towards the ___.

Answer 21

reactants.
products.

Question 22

What is the equation for:
1. Total entropy change?
2. Entropy change of a system?
3. Entropy change of the surroundings?

Answer 22

1. system + surroundings - ΔS(Total) = ∑ΔS (system) + ∑ΔS (surroundings)
2. Products - reactants - ΔS(system) = ∑ΔS (products) - ∑ΔS (reactants)
3. ΔS(surroundings) = -ΔH/T
   * -ΔH/T is just the T = ΔH/ΔS equation, but rearranged for S, and inverted because it’s for the surroundings not the system (whatever happens to the system, the opposite happens to the surroundings - push and pull theory

Question 23

For a feasible reaction, the ___ entropy must ___.

Answer 23

total, increase

Question 24

ΔS (Total) must be _ve for a spontaneous reaction, but the ΔS of the system or surroundings can be _ve, as long as ΔS (Total) is still _ve.

Answer 24

+ve, -ve, +ve

Question 25

In the equation ΔG = ΔH - TΔS, ΔS is of the ___, which means it can be _ve and the reaction can still be ___.

Answer 25

system, -ve, feasible